Rosh Hashanah marks the start of the civil year in traditional Jewish reckoning.
It begins on 1 Tishrei, the first day of Tishrei in the Hebrew lunisolar calendar. On that date, the Hebrew year number advances, and the day begins at sunset because Jewish calendar dates run from evening to evening. Rosh Hashanah is therefore often called the Jewish New Year.
Jewish tradition also recognizes other “new years” for distinct purposes. The first of Nisan begins the ecclesiastical or festival year and is the month in which Passover occurs. Tu BiShvat is the new year for trees, while other dates historically served agricultural or tithing purposes. These multiple reckonings explain why Tishrei is the seventh month in one sense but the beginning of the civil year in another.
“Civil” here does not mean secular or nonreligious. It means the ordinary calendar year used for year numbering and several legal, agricultural, sabbatical, and jubilee calculations.